1. Field of the Invention
The present invention relates to communication systems, communication cards, and communication methods, and particularly to a communication system that performs communication in a network, a communication card that performs communication in a network, and a communication method for performing communication in a ring network having a redundant structure made by a resilient packet ring (RPR).
2. Description of the Related Art
Information communication networks have been changing from local area networks (LANs) in companies and houses to networks in entire urban areas, that is, wider-area networks. For example, a plurality of Ethernet (registered trademark) LAN environments are connected by the use of Layer-2 switches to form an integrated wide-area 10-gigabit Ethernet (10 GbE), which has been widespread.
Main technologies of information transfer in wide-area networks, including 10 GbEs, include a synchronous optical network/synchronous digital hierarchy (SONET/SDH). The SONET/SDH multiplexes low-speed channels hierarchically to provide higher-speed channels and specifies an interface for effectively multiplexing various types of communication services. The SONET/SDH has been standardized and developments thereof have been advancing. As a topology for wide-area networks, ring networks, where a plurality of nodes are connected in a ring manner, are mainly used.
Currently, SONET/SDH-based ring networks are mainly used as communication backbones of wide-area networks that perform long-distance transfer. A technology called a resilient packet ring (RPR) has recently attracted attention, as a substitute for the SONET/SDH.
The RPR is a new media-access-control (MAC) frame transfer technology which has been being standardized as IEEE 802.17 (its protocol is on a Layer-2 MAC sub-layer like the Ethernet). The RPR does not depend on Layer 1 (uses existing techniques of Layer 1) and implements ring topology.
The RPR can use a transmission-rate series of optical carrier “n” (OC-n) in the SONET or synchronous transport module “n” (STM-n) in the SDH, or a Layer-1 physical layer, which includes 10 GbEs, to transfer IEEE 802.17 MAC frames (RPR frames) in a ring network (RPR over SONET/SDH and RPR over GbE are possible).
FIG. 6 shows the structure of an RPR network 100. The RPR network 100 includes nodes 101 to 106, and is a ring network in which the nodes 101 to 106 are connected in a ring manner by optical fibers. Information moving around the ring network can be dropped to a tributary side, or information is added from a tributary side to the ring network, through the nodes 101 to 106.
The RPR network 100 has a double ring which allows packets to flow in two directions opposite each other. In the figure, packets flow along a ring route F1 clockwise and flow along a ring route F2 counterclockwise. Information is transferred and distributed in units of packets in the RPR whereas information is transferred and distributed in units of streams formed of a plurality of OC or STM channels in the SONET/SDH.
The RPR transfers packets by spatial reuse. Spatial reuse will be described by comparing it with a unidirectional path switched ring (UPSR), which is one operation form of conventional SONET rings.
FIG. 7 is a concept view of the operation of a UPSR. Nodes 111 to 114 are connected in a ring shape to form a ring network. The UPSR is an operation form that avoids a failure by sending data for the current system in one direction of the ring while the data is always sent in the opposite direction for the reserved system and by witching to the reserved system if a failure occurs on the current system.
When the node 114 sends data to the node 111, for example, the node 114 sends the data through a current-system line W and, at the same time, always sends the same data in the opposite direction via a reserved-system line Pr through the nodes 113 and 112 to the node 111 (the node 111 selects, in a normal operation, the data coming from the WEST).
If a line failure occurs in the current-system line W, the line is switched to the reserved-system line Pr to immediately avoid the failure. Since the reserved-system line Pr through the nodes 113 and 112 is not relevant to actual communication in a normal operation, it uses a space wastefully (because it provides time division multiplexing (TDM) transfer, it reduces time slots that can be used in a normal operation).
FIG. 8 shows spatial reuse. Spatial reuse is a function for transferring data in a ring at the shortest path in a normal operation of a network having the ring. As shown in FIG. 8, when each of nodes 101 to 106 transfers data to adjacent nodes in a network 100a, spaces (spans Sp1 to Sp6) are only used between the transmission-side nodes and the receiving-side nodes at the shortest paths.
Assuming that the node 105 sends a packet to the node 106, for example, the node 105 uses only a path P1 in a span Sp5 to send the packet, and does not send it round through a redundant route as in the UPSR. Therefore, with the use of the same transfer bandwidth, communication is allowed between nodes by using spans Sp1 to Sp4 and Sp6, other than the span Sp5. In this way, since the RPR uses spatial reuse, which transfers packets at required zones only, the transfer bandwidth can be effectively used.
As failure-remedy methods in the RPR, IEEE 802.17 defines Wrapping remedy, in which data is sent back at the point where a failure occurs to avoid the failure, and Steering remedy, in which, when a failure is detected, each node calculates paths again to avoid the failure. It is determined that the time required to switch the path between a failure and its recovery is 50 milliseconds or less, which is similar to that in the SONET/SDH.
The RPR also has a FairRate (Fairness) function, which dynamically adjusts the transmission rate of each node. The FairRate function will be described below by referring to FIG. 9. The FairRate function dynamically adjusts the transmission rate of each node according to the traffic state in the entire ring to allow each node to use the ring bandwidth in a fair manner.
In FIG. 9, when a buffer of a node 101 reaches a congestion level and the node 101 detects the congestion in a network 100, the node 101 reports the bandwidth to be obtained to a node 102, which is the next upstream node of the node 101, via the ring opposite to the ring which is in the congestion state. When the node 102 receives the report, it adjusts its own use bandwidth so as not to exceed the bandwidth to be obtained by the node 101. The node 102 also reports the bandwidth to be obtained, to a further upstream node.
Such control is performed at each node in the ring to dynamically adjust the transmission rate of each node to maintain fairness in bandwidth.
As described above, the RPR has features such as effective use of bandwidth with the use of spatial reuse, acquisition of bandwidth fairness with the use of FairRate algorithm, and failure recovery within 50 milliseconds similar to that in the SONET/SDH. It has been highly expected that the RPR can form high-quality, highly reliable networks which can cover various media.
As a conventional technology for increasing the bandwidth of a network, a method for switching a ring node so as to couple with another path of another network in order to increase the transfer capacity has been proposed (for example, at paragraphs [0014] to [0036] and FIG. 1 in PCT International Patent Application Publication No. 2002-510160).
In the standard defined by IEEE 802.17, each node of an RPR ring network sends packets at the same transfer rate. For example, both ring routes F1 and F2 have the same packet transfer rate in FIG. 6, and the same packet transfer rate is used in communication at each of the spans Sp1 to Sp6 in FIG. 8.
In actual network operations, however, uniform traffic occurs in a few cases. When a data center to which a server applies centralized control is connected to a node on an RPR ring, or when headquarters or a large city is located at a specific position in an RPR ring, for example, traffic concentrates at a specific node.
FIG. 10 is a view showing the state of a network 100b where a load concentrates on a specific node. The network 100b differs from the network 100 shown in FIG. 6 in that a server 101a performing centralized control is connected to a node 101.
Packets added at nodes 102 to 106 are all transferred to and dropped at the node 101. In an optical fiber, which is a physical transfer path, at a span Sp6 between the nodes 101 and 106, three logical paths are provided, and the bandwidth of the optical fiber is close to its limit.
As described above, since an RPR ring network provides the same transfer rate anywhere, even if a local bandwidth needs to be increased, it is necessary to increase the bandwidth in the same way at all spans.
When a ring is configured with a transfer capacity of 100 Mbps, for example, if it is necessary to provide a transfer capacity of 200 Mbps for an optical fiber at the span Sp6 in the clockwise direction, the bandwidth should also be increased to 200 Mbps in all optical fibers at the spans Sp1 to Sp5 (in this case, the bandwidth should be increased to 200 Mbps also in the counterclockwise direction).
Then, a portion where a transfer capacity of 100 Mbps is sufficient, such as a path P2 in the span Sp5 is configured to have a transfer capacity of 200 Mbps. This means that the conventional RPR network operations are not efficient.
When traffic is increased locally to approach the limit of the transfer bandwidth, if a transmission request exceeding the physical transfer bandwidth is generated, the conventional RPR needs to configure an RPR network having a greater capacity. To increase the bandwidth at a specific span, it is actually necessary to install a new optical interface card having a large capacity in the entire RPR ring network having a maximum of 255 nodes. The time and cost required for this system configuration are enormous, and it is very inefficient.